The invention relates to an apparatus and method for aligning a test surface to an interferometer axis without using any physical indicia, particularly a reticle formed on a diffuse plate.
U.S. Pat. No. 4,201,473 (Domenicali et al.) issued May 6, 1980 describes an interferometer in which a diffuse screen has thereon an integral alignment reticle physically located at the focus of wavefronts reflected by test and reference surfaces to be aligned with an optical axis of the interferometer. During alignment of commercial embodiments of this interferometer, the optics are configured so that a spot of light which represents the image of the test surface to be aligned is focused onto the diffuse screen. The surface of the diffuse screen itself is uniformly illuminated so that the reticle thereon can be viewed. The spot appearing on the diffuse screen then is imaged onto the lens of a video camera together with an image of the illuminated diffuse screen and the opaque reticle thereon. The spot produced by the test surface to be aligned, the surface of the diffuse screen and the reticle are viewed on a video monitor connected to the output of the camera. When the test surface has been adjusted so that the monitor image of the spot is aligned with the monitor image at the marked center of the reticle, the test surface is aligned. A part of the optics including the diffuse screen then is switched out of the path to the lens of the video camera, so the video camera can be used to view an interference pattern produced by beams reflected from a reference surface and the test surface.
The assignee of the present application has utilized a different alignment system in a product known as the WYKO 6000. This alignment system contained no integral reticle or any other physical indicia anywhere on the optical axis of the interferometer. Instead an electronically-generated alignment indicia was produced at the surface of a video monitor. No integral reticle was provided on a diffuse screen, but spot images of test and reference surface were imaged onto a diffuse screen. The diffuse screen was not illuminated and therefore could not be viewed on the video monitor. The spot images on the diffuse screen were viewed on a video camera. The color of the software-generated alignment indicia on the monitor could be different than the imaged spots as they appeared on the monitor screen, and were essentially "transparent" so that the spot images appeared clearly even if located "behind" a part of the software-generated alignment indicia.
It would be more convenient to provide an alignment apparatus and technique which does not utilize either electronically-generated indicia or a physical reticle formed on a lighted diffuse screen and located at a predetermined axial location.